In an optical communication system, an optical fiber transmission line is used as an optical transmission line to transmit signal light. In addition, in signal light transmission (particularly, digital coherent transmission) in the optical communication system, it is demanded to improve an optical signal-to-noise ratio (OSNR). For this reason, it is required to reduce transmission loss and non-linearity of an optical fiber configuring at least a part of the optical fiber transmission line. To reduce the non-linearity of the optical fiber, it is effective to increase an effective cross-sectional area of the optical fiber. For example, an optical fiber in which an effective cross-sectional area has increased to 110 μm2 or more is known.
However, in the optical fiber in which the effective cross-sectional area has increased, confinement of propagation light (fundamental mode light) to a core is weakened. In this case, microbending loss tends to increase. As a result, OSNR may be deteriorated. The microbending loss is the phenomenon that transmission loss increases due to a leak of light in which a fundamental mode of propagation of a core is coupled to a cladding mode and it is known that the cause of the microbending loss is random microbending occurring due to lateral stress application to the optical fiber.
Non Patent Document 1 describes the microbending loss. According to the description of Non Patent Document 1, in a single mode optical fiber having a step-type refractive index profile, the microbending loss exponentially increases with respect to a wavelength. In addition, according to the description of Non Patent Document 1, in an optical fiber having a trench-type refractive index profile, the microbending loss is relatively constant with respect to the wavelength, in a situation where the fundamental mode is coupled to a higher mode to be a leaky mode due to the random microbending. The optical fiber having the trench-type refractive index profile has a core, a first cladding, a second cladding, and a third cladding that are arranged sequentially from a center. A refractive index of the core is higher than refractive indexes of the first cladding and the third cladding and a refractive index of the second cladding is lower than the refractive indexes of the first cladding and the third cladding.